A million years ago, when I was Service Manager for a General Motors
distributor, the paper work for warranty stuff was complex. Warranty claim forms had to have an intelligible and detailed description of the fault, diagnosis and solution. If you could not satisfy those conditions, and also confirm that the fault was the result of a manufacturing or assembly defect - not paid.
Unfortunately, paperwork and written expression is not the core skill of many mechanics. As a result, many claims forms had to be resubmitted. As indicated by others, the pay rate for warrenty work was not high, so there were many dealers who just didn’t bother with it.